ABSTRACT Deep venous thrombosis (DVT) and secondary pulmonary embolism (together, VTE) affect 900,000 Americans, and result in >60,000 deaths each year. A growing body of evidence supports the concept of crosstalk between inflammation and coagulation which amplifies the thrombo-inflammatory response in VTE. Activated, injured, and dying cells release nucleotides that form purinergic halo of ?danger? signals that disrupt vascular homeostasis. Located on the surface of leukocytes and the endothelium, the vascular ectonucleotidase, CD39 rapidly phosphohydrolyzes ATP, and ADP to extinguish these ?danger? signals and promote homeostasis. We have found that CD39 is a potent suppressor of the thrombo-inflammatory response in DVT. Our new preliminary data show that CD39 inhibits circulating leukocyte-platelet interactions, and restricts activation of programmed innate immune responses, including inflammasome assembly and neutrophil extracellular trap (NET) formation, key drivers of the growing thrombus. The hypothesis driving this work is that CD39 provides a critical vascular checkpoint at the intersection of innate immunity and thrombosis to arrest the relentless venous thrombo-inflammatory cycle. Using unique cell lineage (myeloid, neutrophil, endothelial)-specific CD39 gene-deleted mice we have generated, and complementary models of DVT, we will: 1) Elucidate the role for CD39 on cellular recruitment during venous thrombogenesis and maturation. These experiments will define the effect of lineage-specific CD39 on blood cell recruitment, and the spatial dynamics of their interactions during venous thrombogenesis; 2) Determine the mechanism(s) by which myeloid CD39 protects against venous thrombo-inflammation. These experiments will determine the contributions of macrophage and neutrophil CD39 to venous thrombosis. Complementary in vitro and in vivo studies with genetically-modified mice will elucidate the molecular signaling processes by which CD39 inhibits inflammasome activation during thrombogenesis; 3) Determine the effect of CD39-deficiency on microvesicle phenotype, function, and kinetics, during venous thrombosis. These studies will reveal the functional consequences of altering global- and lineage-specific CD39 on microvesicle-mediated thrombo-inflammatory signaling. These studies should yield new insights into how an ectonucleotidase that functions as a checkpoint at the intersection of thrombosis and inflammation may be exploited to improve treatments in venous thrombosis.